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Maxim > Design Support > Technical Documents > Application Notes > Oscillators/Delay Lines/Timers/Counters > APP 2097
Keywords: VCXO, MPEG2 CLK, AC-3 CLK, Audio DAC CLK, Jitter, Pulling range, MAX9485, STB CLK,
DTV CLK
APPLICATION NOTE 2097
Using a VCXO (Voltage-Controlled Crystal
Oscillator) as a Clock (CLK) Generator
Aug 24, 2004
Abstract: A VCXO (voltage controlled crystal oscillator) is an oscillator whose frequency is determined by
a crystal, but can be adjusted by a small amount by changing a control voltage. VCXO clock (CLK)
generators have been used in various applications, such as digital TV, digital audio, ADSL, and STB.
This application note provides the general structure for a VCXO CLK generator, key performance
measurements, guidelines for PCB design, and a test setup for MAX9485, a VCXO CLK Gen chip for
MPEG2 and AC-3 audio applications.
Structure and Application of a VCXO CLK Generator
A VCXO, or voltage controlled crystal oscillator, oscillates with a frequency determined by a crystal, but
which can be adjusted over a narrow range by a control voltage, usually in the range of 0V–2V or 0V–
3V. The tuning range of the VCXO is about ±100ppm to ±200ppm. Figure 1 shows the structure of a
typical VCXO CLK generator and the crystal's circuit model.
Figure 1. Block diagram of a typical VCXO CLK generator.
As the capacitance of the varactors CV1 and CV2 changes, the crystal's model is affected and the
oscillation frequency changes. The two shunt external shunt capacitors CS1 and CS2 are used to adjust
the tuning range and offset the center frequency. Mathematically, according to the crystal circuit in Figure
1, the resonating frequency of the crystal can be expressed as
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